expression: fix panic of explain for connection when plan cache is enabled (#16206)

executor/executor.go

@@ -1551,6 +1551,9 @@ func ResetContextOfStmt(ctx sessionctx.Context, s ast.StmtNode) (err error) {
 		sc.InExplainStmt = true
 		s = explainStmt.Stmt
 	}
+	if _, ok := s.(*ast.ExplainForStmt); ok {
+		sc.InExplainStmt = true
+	}
 	// TODO: Many same bool variables here.
 	// We should set only two variables (
 	// IgnoreErr and StrictSQLMode) to avoid setting the same bool variables and

executor/explainfor_test.go

@@ -16,11 +16,14 @@ package executor_test
 import (
 	"crypto/tls"
 	"fmt"
+	"math"
 
 	. "github.com/pingcap/check"
 	"github.com/pingcap/parser/auth"
 	"github.com/pingcap/tidb/planner/core"
+	"github.com/pingcap/tidb/session"
 	"github.com/pingcap/tidb/util"
+	"github.com/pingcap/tidb/util/kvcache"
 	"github.com/pingcap/tidb/util/testkit"
 )
 
@@ -119,3 +122,35 @@ func (s *testSuite) TestExplainMetricTable(c *C) {
 	tk.MustQuery("desc select * from information_schema.cluster_log where type in ('high_cpu_1','high_memory_1') and time >= '2019-12-23 16:10:13' and time <= '2019-12-23 16:30:13'").Check(testkit.Rows(
 		`MemTableScan_5 10000.00 root table:CLUSTER_LOG start_time:2019-12-23 16:10:13, end_time:2019-12-23 16:30:13, node_types:["high_cpu_1","high_memory_1"]`))
 }
+
+func (s *testSuite) TestExplainForConnPlanCache(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	orgEnable := core.PreparedPlanCacheEnabled()
+	defer func() {
+		core.SetPreparedPlanCache(orgEnable)
+	}()
+	core.SetPreparedPlanCache(true)
+	var err error
+	tk.Se, err = session.CreateSession4TestWithOpt(s.store, &session.Opt{
+		PreparedPlanCache: kvcache.NewSimpleLRUCache(100, 0.1, math.MaxUint64),
+	})
+	c.Assert(err, IsNil)
+
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int)")
+	rows := tk.MustQuery("select connection_id()").Rows()
+	c.Assert(len(rows), Equals, 1)
+	connID := rows[0][0].(string)
+	tk.MustExec("prepare stmt from 'select * from t where a = ?'")
+	tk.MustExec("set @p0='1'")
+	tk.MustExec("execute stmt using @p0")
+	tkProcess := tk.Se.ShowProcess()
+	ps := []*util.ProcessInfo{tkProcess}
+	tk.Se.SetSessionManager(&mockSessionManager1{PS: ps})
+	tk.MustQuery(fmt.Sprintf("explain for connection %s", connID)).Check(testkit.Rows(
+		"TableReader_7 8000.00 root  data:Selection_6",
+		"└─Selection_6 8000.00 cop[tikv]  eq(cast(test.t.a), 1)",
+		"  └─TableFullScan_5 10000.00 cop[tikv] table:t keep order:false, stats:pseudo",
+	))
+}

expression/constant.go

@@ -108,11 +108,11 @@ func (c *Constant) Clone() Expression {
 
 // GetType implements Expression interface.
 func (c *Constant) GetType() *types.FieldType {
-	if c.ParamMarker != nil {
+	if p := c.ParamMarker; p != nil && !p.ctx.GetSessionVars().StmtCtx.InExplainStmt {
 		// GetType() may be called in multi-threaded context, e.g, in building inner executors of IndexJoin,
 		// so it should avoid data race. We achieve this by returning different FieldType pointer for each call.
 		tp := types.NewFieldType(mysql.TypeUnspecified)
-		dt := c.ParamMarker.GetUserVar()
+		dt := p.GetUserVar()
 		types.DefaultParamTypeForValue(dt.GetValue(), tp)
 		return tp
 	}
@@ -176,8 +176,13 @@ func (c *Constant) VecEvalJSON(ctx sessionctx.Context, input *chunk.Chunk, resul
 }
 
 func (c *Constant) getLazyDatum() (dt types.Datum, isLazy bool, err error) {
-	if c.ParamMarker != nil {
-		dt = c.ParamMarker.GetUserVar()
+	if p := c.ParamMarker; p != nil {
+		if p.ctx.GetSessionVars().StmtCtx.InExplainStmt {
+			// Since `ParamMarker` is not nil only in prepare/execute context, the query must be `explain for connection` when coming here.
+			// The PreparedParams may have been reset already, to avoid panic, we just use the pre-evaluated datum for this constant.
+			return dt, false, nil
+		}
+		dt = p.GetUserVar()
 		isLazy = true
 		return
 	} else if c.DeferredExpr != nil {